One good reason to support any English team in European competitions is that it helps the overall EU rating of every other team in the English leagues.
Here is an explanation of how this is achieved.
http://www.eurofootsie.com/modules.php?op=modload&name=Footie&file=description
Here are the ratings,
http://www.eurofootsie.com/index.php
My guess is that betting organisations needed some way of ranking all the European clubs who were likely to get a chance to play in the big European competitions, but then it’s usefulness was latched on to by UEFA itself to decide how playoff groups should be organised.
It is a matter of commercial importance that the big money generating teams in Europe do not get knocked out of these tournaments too early or huge numbers of fans will lose interest.
By ranking these clubs it is possible to ensure that the big teams are more likely to meet in the later stages, and less so in the earlier stages.
Each European nation football federation has a certain mumber of EU competion places available to their teams but of course some leagues are very much more competitive than others, and some leagues are far bigger money spinners than others.
For instance, although Wales is entitled to send a team or two, even the best of them is nothing like as strong as the weakest of say the Italian candidates.
The EU ranking of teams ensures that these relative minnows that bring less money into UEFA’s coffers will soon be exiting the tournaments, but it does give thise small teams a shot at glory and glamour.
If the teams belonging to English leagues do very well in European tournaments then they obviously get rated higher in the EU standings, but it has the knock on effect of increasing the rating of all those clubs in the English domestic leagues, after all if say Man U get beaten by say Tottenham then it is argued that one of the best clubs in Europe have been beaten and the team that did so should be rated accordingly.
If you look at those Euro ratings you will see that there are six Spanish clubs in the top ten and this is partly an effect of them having to play the European champions during their own domestic season several times.